Mechanical rotary shaft seals, also referred to as mechanical shaft seals, are among the most important parts of a pump system. In most cases, the mechanical rotary shaft seal is the barrier between the surrounding environment and the fluid being pumped. In the case of a subsea pump, the mechanical shaft seal is the barrier between the motor cooling fluid, also referred to as the barrier fluid, and the production fluid, e.g., oil, water, gas and optional chemicals and inhibitors. The barrier fluid used in the subsea pump system serves a number of purposes. Barrier fluid cools the pump motor, works as an insulator between the motor housing and the motor stator, prevents production fluid ingress into the motor, and lubricates the bearings and the mechanical shaft seal. If the barrier fluid is contaminated by production fluid, a catastrophic pump system failure such as a motor, mechanical shaft seal and/or bearing failure could occur. A significant portion of all topside and onshore pump failures are caused by mechanical shaft seal failure. This is especially problematic for subsea pump systems.
The operation of known mechanical shaft seals includes the use of a spring mechanism that regulates the distance between the face of the stationary part of the mechanical seal that faces the rotating part of the mechanical seal. This distance or gap is a critical part of the mechanical seal and is at the heart of the seal. This gap generally has a thin lubricating film of fluid therein, either the pumped fluid and/or a barrier fluid. When the pump is started up initially as well as at shutdown and at other times when the pump is running, friction in this gap can become critical. The film thickness and the gap is the function of pump speed and compression force applied by the spring of the mechanical seal. When the pump is stopped the lubrication film thickness becomes nearly zero. Most mechanical shaft seal wear and tear happens at pump start-up. Late in the life of the pump, due to wear, the friction increases between the two mechanical shaft seal faces. The friction generates heat which may cause the mechanical seal to deform leading to accelerated deterioration on the seal surfaces and seal failure.
There exists a need for a mechanical shaft seal for use in pump or compressor systems that avoids or minimizes the aforementioned problems with known seals thereby extending the life of mechanical seals and pumps and compressors using them.